On the overdrawing of melt-spun isotactic polypropylene tapes

The morphol. and mech. properties of melt-spun and post-drawn isotactic polypropylene (iPP) tapes have been studied in order to examine their dependence on the post-draw ratio applied. Special attention is focused on the characterization of the so-called overdrawing behavior of the tapes; at a certain draw ratio, a change of the optical appearance of the tape from transparent to opaque is obsd. Overdrawing is accompanied with changes in the mech. properties, surface, and morphol. of the tapes. For post-draw ratios without overdrawing, and for highly overdrawn tapes, the variation in the mech. properties measured is very small, but for slightly overdrawn tapes, a large scattering of mech. properties has been measured. In the latter case, this behavior is related to the start of internal delamination of the tapes during post drawing. It is shown that strongly overdrawn tapes have excellent mech. properties, in particular, high specific stiffness and strength.